This invention relates to a limited slip differential and more particularly to a limited slip differential, suibable for use in passenger cars. It has heretofore been customary for a long period of time that the so-called selective system is mainly used in four-wheel passenger cars wherein the driver operates by means of a switch a rigid four wheel drive for direct connection between the front and rear wheel shafts and (4 WD) two wheel drive (2 WD) for driving only one of the wheel shafts by releasing the aforesaid connection. However, along with the fact that the merit of four-wheel drive vehicle for use on the usual road has been recognized as important, the drawbacks of the selective system have also become recognizable. Such drawbacks are concerned with the problem of tight-turn braking. Namely at the time of turning the car, the front wheels and rear wheels depicts circle of different radii, thus causing different distances of their movements so that the difference in rotation of the wheels cannot be avoided. In order to avoid such inconvenience, it becomes necessary to incorporate differential gears between the front and rear wheel shafts. In this way, shifting operation of 2 WD and 4 WD may be dispensed with. However, in the case of differential gears consisting of usual bevel gears or planetary gears, the structure concerned is such as to delay the rotation of the resistance-loaded shaft so that when counter force on one-side output shaft becomes less, the shaft will rotate idly until relative driving force is lost. Therefore, in order to prevent the aforesaid condition assuredly in the case of the center-differential 4 WD, such a mechanism becomes necessary as a differential lock or limited slip system. Yet, the center-differential 4 WD becomes complicated in construction and moreover, there occurs the drawbacks of a braking phenomenon at the time of locking.
The Torsen differential system is one of the means which are considered to be capable of solving the above-described problems. The Torsen differential system is one type of differential that employs a worm gear. In the Torsen differential system, a worm is meshed with a worm wheel which has a relatively large diameter and which rotates in a direction perpendicular to the direction of rotation of the worm to limit the differential motion in accordance with the running condition of the vehicle by making good use of the irreversibility of the worm that, when the worm is rotated, the worm wheel is rotated smoothly, whereas, when the worm wheel is activated to rotate so as to turn the worm, the worm wheel can hardly rotate. The structure and operation of the Torsen differential system will be briefly described below. Worm wheels are rigidly secured to two outputs shafts, respectively, and worms are meshed with the respective worm wheels, the worms being meshed with each other through a spur gear. Driving power is applied to the pair of worms. When the speeds of rotation of the two output shafts are equal to each other, the worms do not rotate but force the worm wheels to rotate shafts in opposite directions with substantially the same level of rotational force, but when the vehicle turns a corner at low speed, the worms rotate so as to allow the output shafts to rotate differentially. However, when large reacton force acts on only one output shaft, that is, the corresponding worm wheel alone, the irreversibility of the worm gear functions to suppress generation of a large rotational speed difference between the two output shafts. The degree of differential motion is adjusted in accordance with the relationship between the level of driving power applied to the worms, the level of the reaction force acting on the output shaft and the level of the force from the worm wheels at which the worms are rotated.
The Torsen differential performs an effective limitation on differential motion with the above-described structure. It suffers, however, from the disadvantages that the overall size of the apparatus is unfavourably large and this unavoidably increases the weight and production cost of the differential system.